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Versions: 00 01 02

Internet Engineering Task Force                              S. Cheshire
Internet-Draft                                                Apple Inc.
Intended status: Informational                                  T. Lemon
Expires: January 15, 2019                            Nibbhaya Consulting
                                                           July 14, 2018


     Service Registration Protocol for DNS-Based Service Discovery
                   draft-sctl-service-registration-02

Abstract

   The DNS-SD Service Registration Protocol uses the standard DNS Update
   mechanism to enable DNS-Based Service Discovery using only unicast
   packets.  This eliminates the dependency on Multicast DNS as the
   foundation layer, which greatly improves scalability and improves
   performance on networks where multicast service is not an optimal
   choice, particularly 802.11 (Wi-Fi) and 802.15.4 (IoT) networks.
   DNS-SD Service registration uses public keys and SIG(0) to allow
   services to defend their registrations against attack.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 15, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Introduction

   DNS-Based Service Discovery [RFC6763] is a component of Zero
   Configuration Networking [RFC6760] [ZC] [I-D.cheshire-dnssd-roadmap].

   This document describes an enhancement to DNS-Based Service Discovery
   [RFC6763] that allows services to automatically register their
   services using the DNS protocol rather than using mDNS.  There is
   already a large installed base of DNS-SD clients that can do service
   discovery using the DNS protocol.  This extension makes it much
   easier to take advantage of this existing functionality.

   This document is intended for three audiences: implementors of
   software that provides services that should be advertised using DNS-
   SD, implementors of DNS servers that will be used in contexts where
   DNS-SD registration is needed, and administrators of networks where
   DNS-SD service is required.  The document is intended to provide
   sufficient information to allow interoperable implementation of the
   registration protocol.

   DNS-Based Service Discovery (DNS-SD) allows services to advertise the
   fact that they provide service, and to provide the information
   required to access that service.  Clients can then discover the set
   of services of a particular type that are available.  They can then
   select a service from among those that are available and obtain the
   information required to use it.

   The DNS-SD Service Registration protocol, described in this document,
   provides a reasonably secure mechanism for publishing this
   information.  Once published, these services can be readily
   discovered by clients using standard DNS lookups.

   In the DNS-Based Service Discovery specification [RFC6763] Section 10
   "Populating the DNS with Information" briefly discusses ways that
   services can publish their information in the DNS namespace.  In the
   case of Multicast DNS [RFC6762], it allows services to publish their
   information on the local link, using names in the ".local" namespace,
   which makes their services directly discoverable by peers attached to
   that same local link.

   RFC6763 also allows clients to discover services using the DNS
   protocol [RFC1035].  This can be done by having a system
   administrator manually configure service information in the DNS, but



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   manually populating DNS authoritative server databases is costly and
   potentially error-prone, and requires a knowledgable network
   administrator.  Consequently, although all DNS-SD client
   implementations of which we are aware support DNS-SD using DNS
   queries, in practice it is used much less frequently than mDNS.  The
   Discovery Proxy [I-D.ietf-dnssd-hybrid] provides one way to
   automatically populate the DNS namespace, but is only appropriate on
   networks where services are already advertised using mDNS.  This
   document describes a solution more suitable for networks where
   multicast is inefficient, or undesirable for other reasons, by
   supporting both offering of services, and discovery of services,
   using unicast.

2.  Service Registration Protocol

   Services that implement the DNS-SD Service Registration Protocol use
   DNS Update [RFC2136] [RFC3007] to publish service information in the
   DNS.  Two variants exist, one for full-featured devices, and one for
   devices designed for "Constrained-Node Networks" [RFC7228].

   Full-featured devices are either configured manually, or use the
   "dr._dns-sd._udp" query [RFC6763] to learn the default registration
   domain from the network.  Using the chosen service registration
   domain, full-featured devices construct the names of the SRV, TXT,
   and PTR records describing their service(s).  For these names they
   then discover the zone apex of the closest enclosing DNS zone using
   SOA queries [I-D.ietf-dnssd-push].  Having discovered the enclosing
   DNS zone, they query for the "_dns-update._udp<zone>" SRV record to
   discover the server to which they should send DNS updates.

   For devices designed for "Constrained-Node Networks" [RFC7228] some
   simplifications are used.  Instead of being configured with (or
   discovering) the service registration domain, the (proposed) special
   use domain name [RFC6761] "services.arpa" is used.  Instead of
   learning the server to which they should send DNS updates, a fixed
   IPv6 anycast address is used (value TBD).  It is the responsibility
   of a "Constrained-Node Network" supporting DNS-SD Service
   Registration Protocol to provide appropriate anycast routing to
   deliver the DNS updates to the appropriate server.  It is the
   responsibility of the DNS-SD Service Registration server on a
   "Constrained-Node Network" to handle the updates appropriately.  In
   some network environments, updates may be accepted directly into a
   local "services.arpa" zone, which has only local visibility.  In
   other network environments, updates for names ending in
   "services.arpa" may be rewritten internally to names with broader
   visibility.





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   The reason for these different assumptions is that "Constrained-Node
   Networks" generally require special egress support, and Anycast
   packets captured at the "Constrained-Node Network" egress can be
   assumed to have originated locally.  Low-power devices that typically
   use "Constrained-Node Networks" may have very limited battery power.
   The additional DNS lookups required to discover a registration server
   and then communicate with it will increase the power required to
   advertise a service; for low-power devices, the additional
   flexibility this provides does not justify the additional use of
   power.

   General networks have the potential to have more complicated
   topologies at the Internet layer, which makes anycast routing more
   difficult.  Such networks may or may not have the infrastructure
   required to route anycast to a server that can process it.  However,
   they can be assumed to be able to provide registration domain
   discovery and routing.  By requiring the use of TCP, the possibility
   of off-network spoofing is eliminated.

   We will discuss several parts to this process: how to know what to
   publish, how to know where to publish it (under what name), how to
   publish it, how to secure its publication, and how to maintain the
   information once published.

2.1.  What to publish

   We refer to the message that services using the DNSSD Registration
   Protocol send as a Registration.  Three types of updates appear in a
   Registration: Service Discovery records, Service Description records,
   and Host Description records.

   o  Service Discovery records are one or more PTR RRs, mapping from
      the generic service type (or subtype) to the specific Service
      Instance Name.

   o  Service Description records are exactly one SRV RR, and one or
      more TXT RRs, both with the same name, the Service Instance Name
      ([RFC6763] section 4.1).  In principle Service Description records
      can include other record types, with the same Service Instance
      Name, though in practice they rarely do.  The Service Instance
      Name MUST be referenced by one or more Service Discovery PTR
      records, unless it is a placeholder service registration for an
      intentionally non-discoverable service name.

   o  The Host Description records for a service are a KEY RR, used to
      claim exclusive ownership of the service registration, and one or
      more RRs of type A or AAAA, giving the IPv4 or IPv6 address(es) of
      the host where the service resides.



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   RFC 6763 describes the details of what each of these types of updates
   contains and is the definitive source for information about what to
   publish; the reason for mentioning it here is to provide the reader
   with enough information about what will be published that the service
   registration process can be understood at a high level without first
   learning the full details of DNS-SD.  Also, the "Service Instance
   Name" is an important aspect of first-come, first-serve naming, which
   we describe later on in this document.

2.2.  Where to publish it

   Multicast DNS uses a single namespace, ".local", which is valid on
   the local link.  This convenience is not available for DNS-SD using
   the DNS protocol: services must exist in some specific unicast
   namespace.

   As described above, full-featured devices are responsible for knowing
   in what domain they should register their services.  Devices made for
   "Constrained-Node Networks" register in the (proposed) special use
   domain name [RFC6761] "services.arpa", and let the DNS-SD Service
   Registration server handle rewriting that to a different domain if
   necessary.

2.3.  How to publish it

   It is possible to issue a DNS Update that does several things at
   once; this means that it's possible to do all the work of adding a
   PTR resource record to the PTR RRset on the Service Name if it
   already exists, or creating one if it doesn't, and creating or
   updating the Service Instance Name and Host Description in a single
   transaction.

   A Registration is therefore implemented as a single DNS Update
   message that contains a service's Service Discovery records, Service
   Description records, and Host Description records.

   Updates done according to this specification are somewhat different
   than regular DNS Updates as defined in RFC2136.  RFC2136 assumes that
   updating is a fairly heavyweight process, so you might first attempt
   to add a name if it doesn't exist, and then in a second message
   update the name if it does exist but matches certain preconditions.
   Because the registration protocol uses a single transaction, some of
   this adaptability is lost.

   In order to allow updates to happen in a single transaction,
   Registrations do not include update constraints.  The constraints
   specified in Section 2.4.2 are implicit in the processing of




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   Registrations, and so there is no need for the service sending the
   Registration to put in any explicit constraints.

2.3.1.  How DNS-SD Service Registration differs from standard RFC2136
        DNS Update

   DNS-SD Service Registration is based on standard RFC2136 DNS Update,
   with some differences:

   o  It implements first-come first-served name allocation, protected
      using SIG(0).

   o  It enforces policy about what updates are allowed.

   o  It optionally performs rewriting of "services.arpa" to some other
      domain.

   o  It optionally performs automatic population of the address-to-name
      reverse mapping domains.

   o  A DNS-SD Service Registration server is not required to implement
      general DNS Update prerequsite processing.

   o  Simplified clients are allowed to send updates to an anycast
      address, for names ending in "services.arpa"

2.3.2.  Testing using standard RFC2136-compliant servers

   It may be useful to set up a DNS server for testing that does not
   implement the Registration protocol.  This can be done by configuring
   the server to listen on the anycast address, or advertising it in the
   _dns-update._udp SRV record.  It must be configured to be
   authoritative for "services.arpa", and to accept updates from hosts
   on local networks for names under "services.arpa" without
   authentication.

   A server configured in this way will be able to successfully accept
   and process Registrations from services that send Registrations.
   However, no constraints will be applied, and this means that the test
   server will accept internally inconsistent Registrations, and will
   not stop two Registrations, sent by different services, that claim
   the same name(s), from overwriting each other.









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2.3.3.  How to allow services to update standard RFC2136-compliant
        servers

   Ordinarily Registrations will fail when sent to any non-Registration
   Protocol server because the zone being updated is "services.arpa",
   and no DNS server that is not a Registration Protocol server should
   normally be configured to be authoritative for "services.arpa".
   Therefore, a service that sends a Registration can tell that the
   receiving server does not support the Registration Protocol, but does
   support RFC2136, because the RCODE will either be NOTZONE, NOTAUTH or
   REFUSED, or because there is no response to the update request (when
   using the anycast address)

   In this case a service MAY attempt to register itself using regular
   RFC2136 DNS updates.  To do so, it must discover default registration
   zone and the DNS server designated to receive updates for that zone,
   as described earlier using the _dns-update._udp SRV record.  It can
   then make the update using the port and host pointed to by the SRV
   record, and should use appropriate constraints to avoid overwriting
   competing records.  Such updates are out of scope for the DNSSD
   Registration Protocol, and a service that implements the DNSSD
   Registration Protocol MUST first attempt to use the Registration
   Protocol to register itself, and should only attempt to use RFC2136
   backwards compatibility if that fails.

2.4.  How to secure it

   Traditional DNS update is secured using the TSIG protocol, which uses
   a secret key shared between the client (which issues the update) and
   the server (which authenticates it).  This model does not work for
   automatic service registration.

   The goal of securing the DNS-SD Registration Protocol is to provide
   the best possible security given the constraint that service
   registration has to be automatic.  It is possible to layer more
   operational security on top of what we describe here, but what we
   describe here improves upon the security of mDNS.  The goal is not to
   provide the level of security of a network managed by a skilled
   operator.

2.4.1.  First-Come First-Served Naming

   First-Come First-Serve naming provides a limited degree of security:
   a service that registers its service using DNS-SD Registration
   protocol is given ownership of a name for an extended period of time
   based on the key used to authenticate the DNS Update.  As long as the
   registration service remembers the Service Instance Name and the key




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   used to register that Service Instance Name, no other service can add
   or update the information associated with that Service Instance Name.

2.4.1.1.  Service Behavior

   The service generates a public/private key pair.  This key pair MUST
   be stored in stable storage; if there is no writable stable storage
   on the client, the client MUST be pre-configured with a public/
   private key pair that can be used.

   When sending DNS updates, the service includes a KEY record
   containing the public portion of the key in each Host Description
   update.  The update is signed using SIG(0), using the private key
   that corresponds to the public key in the KEY record.  The lifetimes
   of the records in the update is set using the EDNS(0) Update Lease
   option.

   The lifetime of the DNS-SD PTR, SRV, A, AAAA and TXT records
   [RFC6763] is typically set to two hours.  This means that if a device
   is disconnected from the network, it does not appear in the user
   interfaces of devices looking for services of that type for too long.

   However, the lifetime of its KEY record should be set to a much
   longer time, typically 14 days.  The result of this is that even
   though a device may be temporarily unplugged, disappearing from the
   network for a few days, it makes a claim on its name that lasts much
   longer.

   This way, even if a device is unplugged from the network for a few
   days, and its services are not available for that time, no other
   rogue device can come along and immediately claim its name the moment
   it disappears from the network.  In the event that a device is
   unplugged from the network and permanently discarded, then its name
   is eventually cleaned up and made available for re-use.

2.4.2.  Registration Server Behavior

   The Registration server checks each update in the Registration to see
   that it contains a Service Discovery update, a Service Description
   update, and a Host Description update.

   An update is a Service Discovery update if it contains

   o  exactly one RRset update,
   o  which is for a PTR RR,
   o  which points to a Service Instance Name
   o  for which an update is present in the Registration.




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   An update is a Service Description update if, for the appropriate
   Service Instance Name, it contains

   o  exactly one "Delete all RRsets from a name" update,
   o  exactly one SRV RRset update,
   o  one or more TXT RRset updates,
   o  and the target of the SRV record update references a hostname for
      which there is a Host Description update in the Registration.

   An update is a Host Description update if, for the appropriate
   hostname, it contains

   o  exactly one "Delete all RRsets from a name" update,
   o  A or AAAA RR update(s)
   o  a KEY RR update that adds a KEY RR that contains the public key
      corresponding to the private key that was used to sign the
      message,
   o  there is a Service Instance Name update in the Registration that
      updates an SRV RR so that it points to the hostname being updated
      by this update.

   A Registration MUST include at least one Service Name update, at
   least one Service Description update, and exactly one Host
   Description update.  An update message that does not is not a
   Registration.  An update message that contains any other updates, or
   any update constraints, is not a Registration.  Such messages should
   either be processed as regular RFC2136 updates, including access
   control checks and constraint checks, if supported, or else rejected
   with RCODE=REFUSED.

   Note that if the definitions of each of these update types are
   followed carefully, this means that many things that look very much
   like Registrations nevertheless are not.  For example, a Registration
   that contains an update to a Service Name and an update to a Service
   Instance Name, where the Service Name does not reference the Service
   Instance Name, is not a valid Registration message, but may be a
   valid RFC2136 update.

   Assuming that an update message has been validated with these
   conditions and is a valid Registration, the server checks that the
   name in the Host Description update exists.  If so, then the server
   checks to see if the KEY record on the name is the same as the KEY
   record in the update.  If it is not, then the server MUST reject the
   Registration with the YXDOMAIN RCODE.

   Otherwise, the server validates the update using SIG(0) on the public
   key in the KEY record of the Host Description update.  If the
   validation fails, the server MUST reject the rejectration rejected



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   with the REFUSED RCODE.  Otherwise, the update is considered valid
   and authentic, and is processed according to the method described in
   RFC2136.  The status that is returned depends on the result of
   processing the update.

   The server MAY add a Reverse Mapping that corresponds to the Host
   Description.  This is not required because the Reverse Mapping serves
   no protocol function, but it may be useful for debugging, e.g. in
   annotating network packet traces or logs.

   The server MAY apply additional criteria when accepting updates.  In
   some networks, it may be possible to do out-of-band registration of
   keys, and only accept updates from pre-registered keys.  In this
   case, an update for a key that has not been registered should be
   rejected with the REFUSED RCODE.

   There are at least two benefits to doing this rather than simply
   using normal SIG(0) DNS updates.  First, the same registration
   protocol can be used in both cases, so both use cases can be
   addressed by the same service implementation.  Second, the
   registration protocol includes maintenance functionality not present
   with normal DNS updates.

   Note that the semantics of using the Registration Protocol in this
   way are different than for typical RFC2136 implementations: the KEY
   used to sign the update in the Registration Protocol only allows the
   client to update records that refer to its Host Description.  RFC2136
   implementations do not normally provide a way to enforce a constraint
   of this type.

   The server may also have a dictionary of names or name patterns that
   are not permitted.  If such a list is used, updates for Service
   Instance Names that match entries in the dictionary are rejected with
   YXDOMAIN.

2.5.  TTL Consistency

   All RRs within an RRset are required to have the same TTL
   (Clarifications to the DNS Specification [RFC2181], Section 5.2).  In
   order to avoid inconsistencies, the Registration Protocol places
   restrictions on TTLs sent by services and requires that Registration
   Protocol Servers enforce consistency.

   Services sending Registrations MUST use consistent TTLs in all RRs
   within the Registration.






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   Registration Protocol servers MUST check that the TTLs for all RRs
   within the Registration are the same.  If they are not, the
   Registration MUST be rejected with a REFUSED RCODE.

   Additionally, when adding RRs to an RRset, for example when
   processing Service Discovery records, the server MUST use the same
   TTL on all RRs in the RRset.  How this consistency is enforced is up
   to the implementation.

2.6.  Maintenance

2.6.1.  Cleaning up stale data

   Because the DNS-SD registration protocol is automatic, and not
   managed by humans, some additional bookkeeping is required.  When an
   update is constructed by the client, it MUST include include an
   EDNS(0) Update Lease Option [I-D.sekar-dns-ul].  The Update Lease
   Option contains two lease times: the Update Lease Time and the
   Instance Lease Time.

   These leases are promises, similar to DHCP leases [RFC2131], from the
   client that it will send a new update for the service registration
   before the lease time expires.  The Update Lease time is chosen to
   represent the time after the update during which the registered
   records other than the KEY record should be assumed to be valid.  The
   Instance Lease time represents the time after the update during which
   the KEY record should be assumed to be valid.

   The reasoning behind the different lease times is discussed in the
   section on first-come, first-served naming Section 2.4.1.  DNS-SD
   Registration Protocol servers may be configured with limits for these
   values.  A default limit of two hours for the Update Lease and 14
   days for the SIG(0) KEY are currently thought to be good choices.
   Clients that are going to continue to use names on which they hold
   leases should update well before the lease ends, in case the
   registration service is unavailable or under heavy load.

   The Registration Protocol server MUST include an EDNS(0) Update Lease
   option in the response if the lease time proposed by the service has
   been shortened.  The service MUST check for the EDNS(0) Update Lease
   option in the response and MUST use the lease times from that option
   in place of the options that it sent to the server when deciding when
   to update its registration.

   Clients should assume that each lease ends N seconds after the update
   was first transmitted, where N is the lease duration.  Servers should
   assume that each lease ends N seconds after the update that was
   successfully processed was received.  Because the server will always



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   receive the update after the client sent it, this avoids the
   possibility of misunderstandings.

   DNS-SD Registration Protocol servers MUST reject updates that do not
   include an EDNS(0) Update Lease option.  Dual-use servers MAY accept
   updates that don't include leases, but SHOULD differentiate between
   DNS-SD registration protocol updates and other updates, and MUST
   reject updates that are known to be DNS-SD Registration Protocol
   updates if they do not include leases.

2.6.2.  Sleep Proxy

   Another use of Service Registration Protocol is for devices that
   sleep to reduce power consumption.

   In this case, in addition to the DNS Update Lease option
   [I-D.sekar-dns-ul] described above, the device includes an EDNS(0)
   OWNER Option [I-D.cheshire-edns0-owner-option].

   The EDNS(0) Update Lease option constitutes a promise by the device
   that it will wake up before this time elapses, to renew its
   registration and thereby demonstrate that it is still attached to the
   network.  If it fails to renew the registration by this time, that
   indicates that it is no longer attached to the network, and its
   registration (except for the KEY in the Host Description) should be
   deleted.

   The EDNS(0) OWNER Option indicates that the device will be asleep,
   and will not be receptive to normal network traffic.  When a DNS
   server receives a DNS Update with an EDNS(0) OWNER Option, that
   signifies that the Registration Protocol server should set up a proxy
   for any IPv4 or IPv6 address records in the DNS Update message.  This
   proxy should send ARP or ND messages claiming ownership of the IPv4
   and/or IPv6 addresses in the records in question.  In addition, proxy
   should answer future ARP or ND requests for those IPv4 and/or IPv6
   addresses, claiming ownership of them.  When the DNS server receives
   a TCP SYN or UDP packet addressed to one of the IPv4 or IPv6
   addresses for which it proxying, it should then wake up the sleeping
   device using the information in the EDNS(0) OWNER Option.  At present
   version 0 of the OWNER Option specifies the "Wake-on-LAN Magic
   Packet" that needs to be sent; future versions could be extended to
   specify other wakeup mechanisms.

   Note that although the authoritative DNS server that implements the
   DNSSD Service Registration Protocol function need not be on the same
   link as the sleeping host, the Sleep Proxy must be on the same link.





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3.  Security Considerations

   DNS-SD Service Registration Protocol updates have no authorization
   semantics other than first-come, first-served.  This means that if an
   attacker from outside of the administrative domain of the server
   knows the server's IP address, it can in principle send updates to
   the server that will be processed successfully.  Servers should
   therefore be configured to reject updates from source addresses
   outside of the administrative domain of the server.

   For Anycast updates, this validation must be enforced by every router
   that connects the CDN to the unconstrained portion of the network.
   For TCP updates, the initial SYN-SYN+ACK handshake prevents updates
   being forged from off-network.  In order to ensure that this
   handshake happens, Service Discovery Protocol servers MUST NOT accept
   0-RTT TCP payloads.

   Note that these rules only apply to the validation of DNS-SD
   registration protocol updates.  A server that accepts updates from
   DNS-SD registration protocol clients may also accept other DNS
   updates, and those DNS updates may be validated using different
   rules.  However, in the case of a DNS service that accepts automatic
   updates, the intersection of the DNS-SD service registration update
   rules and whatever other update rules are present must be considered
   very carefully.

   For example, a normal, authenticated RFC2136 update to any RR that
   was added using the Registration protocol, but that is authenticated
   using a different key, could be used to override a promise made by
   the registration protocol, by replacing all or part of the service
   registration information with information provided by a different
   client.  An implementation that allows both kinds of updates should
   not allow updates to records added by Registrations using different
   authentication and authorization credentials.

4.  Privacy Considerations

5.  Acknowledgments

   Thanks to Toke Hoeiland-Joergensen for a thorough technical review,
   to Tamara Kemper for doing a nice developmental edit, Tim Wattenberg
   for doing a service implementation at the Montreal Hackathon at IETF
   102, and [...] more reviewers to come, hopefully.








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6.  References

6.1.  Normative References

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [I-D.sekar-dns-ul]
              Sekar, K., "Dynamic DNS Update Leases", draft-sekar-dns-
              ul-01 (work in progress), August 2006.

6.2.  Informative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://www.rfc-editor.org/info/rfc2131>.

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,
              <https://www.rfc-editor.org/info/rfc2136>.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
              <https://www.rfc-editor.org/info/rfc2181>.

   [RFC2931]  Eastlake 3rd, D., "DNS Request and Transaction Signatures
              ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September
              2000, <https://www.rfc-editor.org/info/rfc2931>.

   [RFC3007]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
              Update", RFC 3007, DOI 10.17487/RFC3007, November 2000,
              <https://www.rfc-editor.org/info/rfc3007>.

   [RFC3152]  Bush, R., "Delegation of IP6.ARPA", BCP 49, RFC 3152,
              DOI 10.17487/RFC3152, August 2001,
              <https://www.rfc-editor.org/info/rfc3152>.





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   [RFC6760]  Cheshire, S. and M. Krochmal, "Requirements for a Protocol
              to Replace the AppleTalk Name Binding Protocol (NBP)",
              RFC 6760, DOI 10.17487/RFC6760, February 2013,
              <https://www.rfc-editor.org/info/rfc6760>.

   [RFC6761]  Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
              RFC 6761, DOI 10.17487/RFC6761, February 2013,
              <https://www.rfc-editor.org/info/rfc6761>.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <https://www.rfc-editor.org/info/rfc6762>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [I-D.ietf-dnssd-hybrid]
              Cheshire, S., "Discovery Proxy for Multicast DNS-Based
              Service Discovery", draft-ietf-dnssd-hybrid-08 (work in
              progress), March 2018.

   [I-D.ietf-dnssd-push]
              Pusateri, T. and S. Cheshire, "DNS Push Notifications",
              draft-ietf-dnssd-push-14 (work in progress), March 2018.

   [I-D.cheshire-dnssd-roadmap]
              Cheshire, S., "Service Discovery Road Map", draft-
              cheshire-dnssd-roadmap-01 (work in progress), March 2018.

   [I-D.cheshire-edns0-owner-option]
              Cheshire, S. and M. Krochmal, "EDNS0 OWNER Option", draft-
              cheshire-edns0-owner-option-01 (work in progress), July
              2017.

   [ZC]       Cheshire, S. and D. Steinberg, "Zero Configuration
              Networking: The Definitive Guide", O'Reilly Media, Inc. ,
              ISBN 0-596-10100-7, December 2005.

Authors' Addresses










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   Stuart Cheshire
   Apple Inc.
   One Apple Park Way
   Cupertino, California  95014
   USA

   Phone: +1 408 974 3207
   Email: cheshire@apple.com


   Ted Lemon
   Nibbhaya Consulting
   P.O. Box 958
   Brattleboro, Vermont  05302
   United States of America

   Email: mellon@fugue.com


































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